In relation to CO2 separation when operating fossil-fired power plants, various exhaust gas recirculation systems exist. The purpose of such systems is to be seen in each case as that of simplifying the separation of pollutants, such as CO2 and NOx, in the exhaust gas which is to be discharged to the atmosphere by an increased pollutant concentration allowing the treatment of smaller exhaust gas mass flows. With these systems, as are described in US2008104958A1 or U.S. Pat. No. 6,598,402 B2, for example, a portion of the exhaust gas is mixed with fresh air, cooled and inducted again by the compressor, compressed, and fed again to the combustion chamber as oxygeneous gas. The limit with regard to the potential recirculation rate is achieved when the oxygen proportion in the gas turbine combustion chamber has reached a permissible minimum.
All modern gas turbines, however, now have a cooling fluid supply taking place at different pressure- and temperature levels for cooling the thermally loaded parts of this very gas turbine. The cooling fluid supply is maintained according to the prior art by extraction of compressed air from the compressor. This air, therefore, bypasses the combustion chamber. In this way, a significant proportion of the fresh air which is inducted on the compressor side is taken away from the combustion in the combustion chamber. This leads to a reduction of the possible recirculation rate. Such an initial situation can also be established in the case of a sequentially fired gas turbine generator set, as is described in EP 0 620 362 B1, for example, the disclosure of which is incorporated by reference herein, in which proportions of the air which is inducted on the compressor side has to be taken away from the combustion of the high-pressure combustion chamber and the low-pressure combustion chamber.
A further disadvantage of previous cooling fluid supply systems is that its pressure has to be designed for the most unfavorable case in order to ensure an adequate cooling fluid supply in all operating states. Gas turbines with such cooling fluid systems, however, often run in modes of operation in which a lower cooling fluid pressure would be quite sufficient. This compromises the efficiency.
A further disadvantage of previous cooling fluid supply systems is that the total air which is inducted by the compressor has to be filtered in a quality (freedom of dust, etc.) as is required by the cooling systems with their fine passages of less than one millimeter in diameter. The largest portion of the air, however, would not at all require this filtering quality and the pressure loss inherently associated therewith.